When tail lights are on, backup lights go on

When tail lights are on, backup lights go on. When in reverse, tail lights and backup lights go on. License plate lights go on as expected, turn signals work as expected. Electric trunk release works as expected. Backup lights do not light otherwise. No auxiliary lighting installed, no extra audio equipment either. I can access the harness connection before the wiring goes into the trunk lid, there are five leads. I assume black is ground. How do I narrow down where the problem is, I assume there's a short but I don't know enough to narrow it down where it might be. Thanks.